testing

Layer-based testing substitutes test layers instead of mocks. Build test doubles as Layer implementations, compose them with Layer.mergeAll, and run tests via Effect.runPromiseExit to assert on Exit outcomes. This approach works with TypeScript projects using typed effects.

Yes. Test layers enable testing repository interactions, error paths, and business flows like createOrder by substituting Layer implementations in self-contained test environments without external dependencies.

Layer-based testing composes test doubles as full Layer implementations within Effect's type system, replacing real services end-to-end. Traditional mocks typically stub individual functions. Layers provide stronger composition guarantees and integrate naturally with Effect's dependency model.

Write a failing test first, implement enough code to pass, then refactor. Use Layer to build both production and test service implementations, then merge them with Layer.mergeAll and run via Effect.runPromiseExit to verify Exit outcomes.

Familiarity with typed effects, Layer utilities, and TypeScript generics is required. Understanding how to compose layers and interpret Exit types—Success or Failure outcomes—enables building and asserting on test flows.

Yes. Test layers let you implement error cases explicitly within Layer definitions, then compose and run them through Effect.runPromiseExit. You can assert on both success and failure Exit outcomes to verify error handling.